Keying circuit



Dec. 12, 1950 T. R. BURNIGHT ETAL KEYING CIRCUIT Filed Nov. 15, 1945 @253 5 5%: 3&3.

T. ROBERT BURNIGHT Patented Dec. 12, 1950 KEYING CIRCUIT Thomas Robert Burnight and John F. Clark, Jr., Washington, D. 0.

Application November 13, 1945, Serial No. 628,338

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) Claims.

This invention relates to keying circuits and is particularly directed to providing an electronic switch for keying successfully a transmission C11- cuit carrying high frequenc currents.

Rapid, clean-cut keying of circuits carrying radio frequency currents is dimcult to accomplish because of several complicating factors not encountered in low frequency circuits. Capacitances in the keying apparatus may allow R. F. currents to leak through to the load even when the key is nominally open. Intermittent making and breaking of the R. F. circuit may result in generator instability, since R. F. generators are usually not linear devices and may shift frequency and wave form as a result of changing load conditions. The keying apparatus itself may load the generator with resistance or capacitance. Finally, if the load circuit contains a tank circuit or any other components capable of oscillation under transient shock, a sudden opening of the transmission circuit may not result in immediate reduction of the load current to zero but may instead bring about transient oscillations continuing for an appreciable period of time.

An object of this invention is to provide a means by which a transmission circuit carrying high frequency currents may be keyed at high or low speeds without appreciable variation in the degree of loading of the current source.

Another object of this invention is to provide an electronic switch for high frequency transmission circuits which does not load the current source or pass appreciable leakage currents.

Another object of this invention is to provide an electronic switch which rapidly dissipates residual energy in a load circuit after disconnecting the load circuit from its power source.

The invention will be described with reference to the appended drawing, which is a diagram in schematic and block form showing the invention employed in a receiver application. The transmission circuit of the present invention receives a high frequency alternating input signal from oscillator l and delivers the signal to tank load 2 for application to receiver 3. The operation of auxiliary components 5, 2 and 3 will be described in detail in later paragraphs.

Voltage from oscillator i is applied to the grid of tube 21], which is returned to ground through resistor 22. The cathode of tube 213 is connected to ground through load resistor 23 and is coupled to tank load 2 b condenser 24. The plate of tube 20 is connected to the positive side of D. C. source the negative side of D. C. source it is grounded.

The plate of tube 38 is connected to the plate of tube 20, and the cathode of tube 35 is connected to the cathode of tube 25 The grid of tube 33 is tied directly to the plate of tube 40, which is connected to the positive side of source It through high resistance Al. The cathode of tube Mi is grounded; its grid is connected to the positive side of source "iii through resistance 42. Keying impulse source 15 is shown in block form; it is a device which supplies a negative voltage during those times when the transmission circuit from oscillator i to load circuit 2 is to be interrupted. Source 45 is connected between the grid of tube ii! and ground.

In describing how the invention functions intermittently to interrupt the transmission of energy to load circuit 2, it will first be assumed that no keying impulse is being produced by source 45. In the absence of a keying impulse, tube 2!] functions as a power amplifier of the cathode follower type, receiving on its grid signal voltage from the oscillator I, which is a relatively high impedance source incapable of delivering much power to a load. Signal-frequency voltage appears at the cathode of tube 2!] greatly increased in power level, and is fed through coupling condenser 24 to tank load 2. The power ouput of tube 20 is suflicient to maintain large amplitude oscillations in tank load 2 notwithstanding the energy consumed therefrom by receiver 3. The peak to peak magnitude of the signal frequency component of voltage at the cathode of tube 2B may be of the order of 200 volts; and the average potential at the cathode may be about ninety volts.

Under the conditions now being described, that is, when no keying impulse is coming from source 45, tube 313 in parallel with tube 28 does not affect the operation in any way. Its grid voltage is the same as the plate voltage of tube 49, which is only ten to fifteen volts above ground, since tube 40 conducts in the absence of a keying impulse and a large drop occurs in resistor ii. The grid of tube 30, therefore, is fifteen volts or more negative with respect to its cathode at all parts of the signal voltage cycle and tube 35 passes no plate current.

Now assume that a negative keying impulse is initiated by impulse source 45. The grid. of tube 4% is at once driven below cutoff, plate current ceases to flow in resistor 4i, and the voltage at the grid of tube 30 starts to rise charging the distributed capacity associated therewith. Pres ently, tube 30 begins to draw plate current, and the added drop in cathode resistor 23 causes the cathodes of tubes 20 and 30 to rise in potential also. A state of equilibrium is attained when the grid of tube 30 starts to draw current; this occurs when the cathodes of tubes 26 and 3% are considerably more positive than the maximum voltage attained by the grid of tube 28,. With its cathode at such a high positive potential, tube 29 is at this point wholly blocked. Hence, no energy can pass through tube 28 to tank load 2. Moreover, load circuit 2 is now shunted by the output impedance of tube 36, which is of the order of 300 ohms; consequentl the stored energy in the tank circuit is very rapidly dissipated, and transient oscillations which would otherwise occur in tank circuit 2 are suppressed.

Oscillator l is not afiected by the switching action, since the load on it is not appreciably diiferent whether the transmission circuit is openor closed.

When the keying impulse from source 55 ends, tube 4%] conducts again, plate current flows through resistor 4|, the grid of tube 38 returns to its previous low voltage, tube 32; ceases to draw plate current, and tube 26 resumes normal operation. Energy from oscillator l is again stepped up in power level and fed to tank load 2 as before. The total recovery time is only a few microseconds.

The circuit components with which the invention is incorporated in the drawing areunits of a super-regenerative receiver. This invention is here employed as an electronic switch to suppress the receiver at intervals such as may be necessary to prevent radiation from the receiver from interfering with intermittently-operating radio equipment located nearby.

Oscillator I is a source of quench voltage for super regenerative receiver 3'; it comprises triode tube. is functioning as a Hartley oscillator. Condenser l2 and coil' [3' are connectedin parallel to form a tank. circuit; one side of'the tank circuit is connected to the grid of tube as through the grid leak biasing circuit consisting of condenser l5 and resistor M. inrparallel. The other side of the tank circuit i2, I3 is grounded: The cathode of tube In is connected to tap 55' on coil. !3; the plate of tube HE is by-passed to ground by condenser 15. Plate supply voltage for tube I is provided by source, Te through deg coupling resistor ll. Output voltage from tube It] is .coupled to the grid of tube by condenser 21. The frequency of oscillator i isof a I'll-333i: tude, suitable for quenching a high-frequency super-regenerative receiver; 600 kc./s. mightbe a typical value.

Tankload circuit 2 consists of condenser 2i andcoil 28 in parallel. One side of tanks is grounded, the other is coupled through condenser 24 to the cathode of tube Tank is resonant at the frequency of oscillator l, and it serves to reduce the harmonic content of the pulsating voltageprovided by tube 29 as a plate supply forsuper regenerative receiver 3.

Receiver 3 comprises super-regenerative amplifier tube e and detector diode 69. Voltage from tank circuit 2 is appli d to the plate of tube 5;; through R. F. choke coil 55, which passes current, of the quench frequency readily but oifers high impedance to currents of the radio signal frequencies. Tube 531s connected in a conventional high-frequency super-regenerative amplifier circuit, comprising self-resonant coil 5!, blocking condenser 53, a grid biasing circuit consisting of condenser 59 and resistor 58 in, paral-' lel, and ,R. F. choke coil 54. Radio signals are introduced in coil 5! by inductive coupling from loop 52. The modulation envelope of the voltage in coil 51 is detected by diode 6|] and the resulting video voltage appears across diode load resistor 63 and hence across output terminals 64 and. 65'.

It will beunderstood that the embodiment of this invention herein shown and described is exemplary only, and that the scope of the invention is to be determined from the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What isv claimed is:

1. In combination, a first electron tube having a cathode, an anode, and a grid; power supply means connected between the anode and ground; impedance means connected between cathode and ground comprising an oscillatory network and a direct connected resistance; means for applying alternating input signal to the grid; a second electron tube having a cathode, an anode, and a grid; means connecting theanode of the second tube to the power supply means; means connecting the cathode of the second tube to the cathode of the first tube; and signal input means connected to the grid of thesecond tube for applying a positive potential thereto operative to block the first tube to the input sigby raising its cathode potential and to damp the oscillatory network with the low cathode output impedance of the second tube.

In combination, a first electron tube'having ode, an anode, and a grid; power supply connected between the anode. and ground; means for applying an alternating input signal to the grid; impedance means c nected between the cathode and com g a network resonant to input signal frequencies and a direct connected resis ce; a second electron'tube having a cathode, an anode and grid; meanscon' meeting the anode of the second tube to the power supply means; means connecting. the cathodeof the second tube to the cathode of the first tube; and signal in ut means connected the grid of the second tu e for app in; a, positive potential thereto opera e to his the first tube to the input signal by ra ing its cathode potential and to the reconnt network with the low cath= ode output impedance of the second tube;

3. In combination, a first electron tubehaving a cathode, anode, and a grid; power supply means connected between the anode and ground; impedance means connected between the cathode and ground; alternating voltage generator means connected between the grid and ground; l-cad coupled to cathode and normally deriving alternating current therefrom; a second electron tube having a cathode, an anode, and a grid; means connecting the anode of. the second tube to the power suppl means; means connecting the cathode of the second tube to the cathode of the first tube; means coupled to the grids of the second tube normally operative to maintain a bias voltage thereon; and signal input means connected to the grid. of the second tube for applying thereto a positive potential operative to block the first tube by raising its cath-v ode potential and to shunt the load means with the low cathode output impedance of the second tube.

4. In combination, a first electron tube having a cathode, an anode, and a grid; power supply means connected between the anode and ground; impedance means connected between the cathode and ground; alternating voltage generator means connected between the grid and ground; load means coupled to the cathode and normally deriving alternating current energy therefrom; a second electron tube having a cathode, an anode, and a grid; means connecting the anode of the second tube to the power supply means; means connecting the cathode of the second tube to the cathode of the first tube; a third electron tube having a cathode, an anode, and a grid; means grounding the cathode of the third tube; means connecting the grid of the second tube to the anode of the third tube; impedance means connecting the anode of the third tube to the power supply means; means operative so to bias the grid of the third tube as to render the third tube normally conducting; and signal input means connected to the grid of the third tube for applying a blocking bias thereto operative to block the first tube by raising its cathode potential and to shunt the load means with the low cathode output impedance of the second tube.

5. In combination, first and second cathode loaded electron tubes having their cathodes connected together, an inductance-capacitance res- REFERENCES CITED The following references are of record in the ;file' of this patent:

UNITED STATES PATENTS Name Date Bingley Sept. 5, 1939 Williams Nov. 3, 1942 Number 2,171,536 

